Wound dressing with conformable elastomeric wound contact layer

ABSTRACT

Low adherency wound dressings which comprise a wound facing layer, an intermediate absorbent layer and an outer layer characterized in that the wound facing layer comprises a conformable elastomeric apertured film, the intermediate absorbent layer comprises a conformable hydrophilic foam and the outer layer comprises a continuous moisture vapor transmitting conformable film; materials therefor and processes for the manufacture thereof are described.

The application is a continuation of 07/998,635 filed Dec. 31, 1992 (nowabandoned) which is a continuation of 07/882,356 filed May 7, 1992 (nowabandoned) which is a continuation of 07/432,808 Oct. 18, 1989 (nowabandoned) which is a continuation of 06/791,720 Oct. 25, 1985 (nowabandoned) which is a continuation of 06/345,488 Feb. 3, 1982 (nowabandoned).

The present invention relates to an absorptive wound dressing suitablefor use on burns or other wounds which dressing has a reduced tendencyto adhere to the wound and can act as a bacterial barrier. The presentinvention also relates to the manufacture and use of such dressings.

Burns and other related wounds such as donor sites and the like presenta serious problem in that they tend to produce large amounts of exudatewhich can cause conventional dressings to become saturated or to stickto the wound or even become infected. One method of covering such woundshas been to cover the wound with a material into which new epithelial orfibroblast growth can penetrate. Dressings of this kind are disclosed inU.S. Pat. Nos. 3,526,224, 3,648,692 and 3,949,742.

However such dressings can be-extremely painful to remove and oftenrequire surgical excision. A fundamentally different approach requiringa fundamentally different type of dressing is to employ materials thatare designed to reduce the propensity to adhere to the wound. Dressingsof this kind are disclosed in British Patent No. 439085, French PatentNo. 947609, U.S. Pat. Nos. 3,543,750, 2,923,298 and British Patent No.778813 which later patents cover successfully used materials such asMelolin ("Melolin" is a registered Trade Mark of T. J. Smith and NephewLimited, Welwyn Garden City, Herts., U.K.). One more recent attempt atnon-adherent dressings is U.S. Pat. No. 3,709,221 which discloses adressing having an outer microporous, liquid repellent fibrous layer, aninner macroporous fibrous layer and an absorbent intermediate layerwhich was also envisaged as normally being fibrous. In order to reducethe tendency of this material to adhere to the wound the inner layer hadto be treated with an agent to render it non-wetted by body liquid. Itis now realised that it would be desirable to provide a dressing inwhich the wound facing layer did not require special treatment. As itwill become apparent hereinafter it has now been discovered that byavoiding fibrous materials it is possible to produce a dressing withreduced tendency to adhere to wounds without the need for specialtreatments. An attempt at producing an absorbent dressing is describedin U.S. Pat. No. 3,888,748 which describes a dressing fabricated from atleast four sheet materials. The wound facing part of the dressingapparently consists of a grid or scrim coated with polyethylene in suchmanner that the polyethylene surrounds the filaments of the grid andcollects any loose thread or particle that may be present in the corematerial. It is now realised that it is desirable to avoid the use ofwound facing layers that can allow such penetration of the central layerto the wound surface. It has also been realised that it would bedesirable to provide a material that was highly conformable to the woundso that it is possible to minimise the quantity of exudate between thewound surface and the dressing. U.S. Pat. Nos. 3,709,221 and 3,888,248disclose materials which are bonded along their edges which may reflecta desire to improve conformability. The dressing of the presentinvention allows for bonding over the whole of the operative area whileretaining flexibility.

Accordingly the present invention provides a low adherency wounddressing which comprises a wound facing layer, an intermediate absorbentlayer and an outer layer which wound dressing is characterised in thatthe wound facing layer comprises a conformable elastomeric aperturedfilm, the intermediate absorbent layer comprises a conformablehydrophilic foam and the outer layer comprises a continuous moisturevapour transmitting conformable film.

Normally the three layers of the dressing of this invention are attachedin a contiguous and co-extensive manner; that is the dressing isnormally provided in the form of a laminate.

The conformable elastomeric apertured film of the dressing of thisinvention acts as a low adherency wound facing layer. This layer allowswound exudate to pass to the absorbent layer but prevents the absorbentlayer making direct contact with the wound surface.

Preferably the elastomeric apertured film is sufficiently conformable toallow the wound dressing to conform to the body contours and therebymaintain overall contact with the wound surface to ensure that exudatefrom the wound is absorbed.

It is also desirable that the elastomeric apertured film should besufficiently elastically extensible to adjust to any dimensional changesin the absorbent layer which may occur, for example, expansion on liquiduptake.

Normally the elastomeric apertured film is made of a pharmaceuticallyacceptable water insoluble polymer. Preferred polymers for use areelastomers. Suitable elastomers include polyurethanes, polybutadiene andthe like. The preferred materials for the apertured films arethermoplastic polyurethanes and polybutadienes.

Preferred thermoplastic polyurethanes are linear polyurethanescontaining polyether or polyester groups-Suitable linear polyesterpolyurethanes are disclosed in U.S. Pat. No. 2,871,218. Suitable linearpolyether polyurethanes are disclosed in U.S. Pat. No. 2,899,411.Favoured thermoplastic polyurethanes include Estanes from B. F. GoodrichCorp. Preferred solution casting grades are Estane 5714F1, 5702, and5703. Apreferred extrusion grade is Estane 580201.

Suitable polybutadienes are 1,2 polybutadienes. Favoured 1,2polybutadienes contain a major amount of syndiotactic 1,2 polybutadiene,have a crystallinity of 25% to 30% and an average molecular weight inexcess of 100,000. Preferred 1,2 polybutadienes are known as RB 810,RB820 and RB 830 made by Japan Synthetic Rubber Co.

The number and size of the apertures in the apertured film will besufficient to allow the wound exudate to pass through the film to theabsorbent layer. Most aptly the apertured film is adapted so that thesize of apertures in combination with the thickness of the film preventthe absorbent layer contacting the wound surface. Suitable aperturedfilms have apertures with a dimension of from 0.05 to 4 mm, more aptlyfrom 0.05 to 2.5 or 0.05 to 2 mm and preferably from 0.1 to 2.5 mm.Suitable apertured films have a thickness of 0.01 to 2.5 mm, typically0.01 to to O.25 mm and preferably of 0.05 to 0.5 mm.

Favoured apertured films of the invention have 4 to 40 apertures per cmwith a dimension of 0.05 mm to 2.5 mm.

The conformable apertured film can be in any convenient form such as aperforated film or a net.

In a favoured aspect of the invention the elastomeric apertured film isin the form of a net which is preferably an integral net. The term`intergral net` means a net in which the strands and junctures areformed integrally during manufacture.

The integral net of the wound dressing of the invention can have anyconvenient form depending on the chosen arrangement of strand, junctureand hole areas and also their shapes and relative size.

In one preferred form the net consists essentially of longitudinal andtransverse strands intersecting at right angles to give a square gridhole pattern.

Suitable nets of this type aptly have 2 to 40 strands per cm desirably 4to 40 strands per cm and preferably 2 to 24 strands per cm in bothlongitudinal and transverse directions.

Variations on the square grid pattern can give other desirable forms ofthe integral net. Unequal density of strands in either the longitudinalor transverse directions will give rectangular hole areas. Continuousparallel strands in one direction with a staggered arrangement ofconnecting strands in the other direction will give a "brickwork"pattern. Other apt forms of the integral polymer nets can have strandsat an angle to the longitudinal or transverse direction (that isdiagonal strands). Another preferred form of the integral polymer netcan have a staggered arrangement of circular or approximately circular(for example hexagonal) arrangements of strands and hole areas. Theintegral polymer net can be in the form of a mixed pattern of two ormore of the arrangements if desired.

The apertured film used in this invention aptly will have a weight of 10gsm to 80 gsm and preferably will have a weight of 15 gsm to 50 gsm.

The desirable conformability of the wound dressing of the invention isconsistent with the use of elastomeric materials such as integral netsof polyurethane or other elastomer.

Suitable integral nets of polyurethane or other elastomer will have anelongation at break of 100% to 800%, desirably of 200% to 750% andpreferably of 300% to 700% when measured as a 2.5 cm wide strip at 30cm/min strain rate at 20° C.

Other suitable conformable apertured films are thin flexible elastomericfilms which have been perforated.

The continuous moisture vapour transmitting conformable film outer layerof the wound dressing of the invention may be used to regulate themoisture loss from the wound area under the dressing and also to act asa barrier to bacteria so that bacteria on the outside surface of thedressing cannot penetrate to the wound area.

Suitable continuous conformable films will have a moisture vapourtransmission rate of 300 to 5000 grams preferably 500 to 2000grams/square meter/24 hrs at at 37.5° C. at 100% to 10% relativehumidity difference. It has been found that such moisture vapourtransmission rates of the continuous film allow the wound under thedressing to heal under moist conditions without causing the skinsurrounding the wound to macerate.

This outer layer will be made of polymer.

Suitable polymers for use in the outer layer include urethanes andcopolymers of alkoxy alkyl acrylates or methacrylates such as thosedisclosed in British Patent No. 1,280,631.

Preferably the outer layer is a conformable polyurethane film.

Preferred polyurethane films are made from linear polyurethanes ashereinbefore described with respect to the wound facing layer. Favouredcontinuous films will be 12.5 micron to 37.5 micron thick. A preferredpolyurethane for use in such thickness is Estane 5714F. A 25 micronthick film of Estane 5714F has a moisture vapour transmission rate ofapproximately 1800 g/m² /24 hours/37.5° C. at 100% to 10% relativehumidity difference so that it may be employed to produce a moisturevapour transmission within the preferred range.

The outer layer can be a conformable polyurethane-incompatible polymerblend film continuing voids.

Suitable conformable polyurethane blend films are disclosed in UnitedKingdom patent application No. 8122250.

Apt conformable polyurethane blend film outer layers have a thickness of0.0125 m to 0.125 mm. Such films can have a moisture vapour transmissionrate of at least 500 g and preferably at least 1000 g/m² /24 hours/at37.5° C. at 100% to 10% relative humidity difference.

A preferred polyurethane blend film comprises a blend of a linearpolyurethane (60 parts by weight of Estane 580201 available from B. F.Goodrich) and a high impact polystyrene (40 parts by weight of compoundref. 6 mw available from R. H. Cole Limited). A favoured film of thiscomposition has a thickness of 0.084 mm and a moisture vapourtransmission rate of 1660 g/m² /24 hours/at 37.5° C. at a 100% to 10%relative humidity difference.

The continuous moisture vapour transmitting conformable film outer layercan comprise a moisture vapour transmitting adhesive layer.

In such adhesive containing outer layers the adhesive layer will becovered by an extensible moisture vapour transmitting layer to provide anon adhesive surface on the outer layer of the dressing.

At least one of the adhesive or extensible layers will be continuous.

Preferred adhesive layers are continuous. Suitable adhesives which aremoisture vapour transmitting as a continuous layer include variousacrylate ester copolymers, polyvinyl ethyl ether and polyurethanepressure sensitive adhesives. Examples of suitable pressure sensitiveadhesives are given in British Patent No. 1,280,631.

A preferred pressure sensitive adhesive comprises a blend of a high andlow viscosity polyvinyl ethyl ethers in particular `adhesive compositionA` disclosed in British Patent Specification No. 1,280,631. Otherpreferred pressure sensitive adhesives comprise copolymers of acrylateester with acrylic acid for example as disclosed in United KingdomApplication No. 8106707 and in particular a copolymer of 47 parts byweight of butylacrylate, 47 parts by weight of 2 ethyl hexyl acrylateand 6 parts by weight of acrylic acid with an intrinsic viscosity of atleast 1.9 dl/g polymerised in acetone according to the general methodgiven in above United Kingdom application.

The suitable continuous adhesive layers can have a weight per squaremeter of 15 g to 70 g and preferably of 20 g to 40 g.

The extensible moisture vapour transmitting layer which covers theadhesive layer can be the comformable continuous films hereinbeforedescribed. Alternatively the extensible layer can be a discontinuouslayer. Suitable discontinuous extensible layers include apertured nonwoven fabrics which are extensible in at least one direction.

Preferred extensible apertured non woven fabrics include those made frombonded viscose filaments. Suitable fabrics of this kind are available asBemliese (Trade Mark) from Asahai Chemical Company. Bemliese isavailable in weights per square meter ranging from 18 g to 45 g. Afavoured fabric is Bemliese G204 which has a weight per square meter of18.5 g.

The conformable hydrophilic polymer foam absorbent layer used in thedressing of this invention is adapted to be capable of absorbing thewound exudate e.g. from a burn. It is desirable that the hydrophilicfoam layer absorbs the wound exudate rapidly as this enhances the lowadherency properties of the dressing. Such rapid absorption preventspooling of exudate between the dressing and the wound and it has beenfound that this prevention of pooling is desirable.

Suitable conformable hydrophilic foams will normally be flexible, opencell foams.

The ability of open cell foams to absorb and retain fluids depends tosome extent on the size of foam cells and the porosity of the foam.

Suitable open cell hydrophilic foams of dressings of the invention havea cell size of of 30 microns to 700 microns and preferably a cell sizeof 50 microns to 500 microns. Apt open cell hydrophilic foams ofdressings of the invention have 20% to 70% and preferably 30% to 60% ofthe total membrane area of the cells as membrane openings. Such opencell foams permit transport of fluid and cellular debris into and withinthe foam.

Apt foams may be polyurethane, carboxylated butadiene styrene rubber,polyacrylate or the like foam. Such foams may be made of hydrophilicmaterials per se or may be treated to render them hydrophilic, forexample with surfactants. It is much preferred to use foams which aremade of polymer which is itself hydrophilic as it has been found thatthe exudate is less likely to coagulate rapidly. The use of such foamsof hydrophilic polymer in dressings of the invention can allow the woundto be maintained in a moist condition even when the exudate produced hasbeen absorbed and removed from the wound surface.

Favoured hydrophilic polymer foams are hydrophilic polyurethane andespecially those which are made of crosslinked hydrophilic polyurethane.Preferred foams can be made by reacting a hydrophilic isocyanateterminated polyether prepolymer with water. Favoured hydrophilicpolyurethane foams of this type include those known as Hypol foams.Hypol foams can be made from Hypol hydrophilic prepolymers marketed byW. R. Grace and Co.

Suitable hydrophilic foam absorbent layers have a thickness of 0.5 mm to20 mm, more suitably 0.8 mm to 15 mm and preferably 1 mm to 12 mm.

The wound dressings of the invention preferably consist of a conformableelastomeric apertured film, an intermediate conformable hydrophilicpolymer foam layer and a continuous moisture vapour transmittingconformable polymer film outer layer in which the layers are attached ina contiguous and co-extensive relationship.

The wound dressing of this invention may be in any convenient form. Apreferred form is a pad of rectangular shape. Suitable sizes of such apad are from 10 cm to 20 cm×30 cm. Another preferred form is an elongatestrip which may be in the form of a roll. Such a strip may be used as abandage or may be used to prepare smaller dressings.

It is desirable that the wound dressing of this invention are sterile.The wound dressing of the invention is advantageously provided inbacteria impervious pouches. Such packed forms can be prepared underaseptic conditions or alternatively sterilised after packing by aconventional procedure. A favoured sterilisation procedure is heatsterilisation, for example by steam. Another favoured procedure isethylene oxide sterilisation or gamma irradiation.

In another aspect the invention provides a process of making a lowadherency wound dressing which comprises bringing together of aconformable elastomeric apertured film layer, an intermediate absorbentlayer comprising a conformable hydrophilic polymer foam and an outerlayer comprising a continuous moisture vapour transmitting conformablefilm.

Normally the bringing together of the layers will be a laminationprocess.

The previously formed individual layers can be formed into a laminate bybonding the layers together in one or more laminating processes.Suitable bonding methods include heat sealing or adhesive bondingproviding the adhesive layer is moisture vapour transmitting.

In a preferred process the foam layer is formed in contact with one orboth of the other layers. This process is favoured as it reduces oreliminates the number of special bonding operations.

In another preferred process the outer conformable film layer is formedon the foam layer for example by spraying a solution of the polymer.

In a continuous process the wound dressing can be made in the form of acontinuous strip which is then cut up into suitable sized dressings.

The conformable hydrophilic polyurethane foam can be made by mixingtogether an isocyanate terminated polyether having functionality of morethan two with a surfactant and water and casting the mixture onto asurface. This surface advantageously may be the outer film of thedressing or the wound facing layer of the dressing. Preferred isocyanateterminated polyethers include Hypols FHP 2000, 2001, 3000, 3001, 2002and 2000HD marketed by W. R. Grace & Co. Hypols are described in abooklet published by W. R. Grace and Co. "Hypol: foamable hydrophilicpolymers-laboratory procedures and foam formulation". Their preparationand use are disclosed in British Patent Specifications No. 1,429,711 and1,507,232.

Suitable surfactants for forming conformable hydrophilic polymer foamsinclude non-ionic surfactants. Favoured non-ionic surfactants areoxypropylene-oxyethylene block copolymers known as Pluronics marketed byBASF Wyandotte. Preferred Pluronics include L64, F87, P38, P75 and L62.Another favoured non-ionic surfactant is a polyoxyethylene stearyl etherknown as Brij 72 marketed by Honeywell Atlas.

To prepare a suitable foam 100 parts by weight of Hypol FHP 2000, 2001,3000, 3001, 2002 or 2000HD is mixed with 0.3 to 7 parts by weight ofsurfactant-or mixtures of surfactants and 30 to 300 parts by weight ofwater and the foaming mixture cast onto a surface. Typical foamingmixtures have a cream time of about 20 secs., a rise time of about 250secs. and a cure time of about 400 secs.

In a continuous process for forming the foam the ingredients are fedinto a continuous mixing and dispensing machine. Suitable conformablehydrophilic polymer foam layers can be made by Casting the foamingmixture before it sets onto a suitable surface by means of a castinghead.

A suitable mixing and dispensing machine is known as Vario-mix suppliedby Prodef Engineering Limited. The foam mix can conveniently bedelivered to the casting head by means of a `fish tail` die.

In a preferred process of forming the dressing in which the foam layeris produced in contact with an external layer it is important that theother external layer should be laminated to the expanded foam while thefoam is still tacky so as to obtain a good bond. Typically 2.5 minutesto 5 minutes, for example 3 mins to 3.5 mins, after the foam has beencast is suitable for bringing the foam into contact with the otherexternal layer.

FIG. 1 illustrates a process of making the conformable integral polymernets of the wound dressing of the invention.

FIG. 2 is a plan view of an embossed pattern sheet casting surface forforming an integral polymer net.

FIG. 3 is a cross-section through line A--A of FIG. 2.

In FIG. 1 a thermoplastic film (1) with an embossed pattern on its uppersurface may be fed from roll (2) to the coating head (3) where asolution (4) may be cast into the recesses of the embossed sheet. Thewet cast net (5) on the embossed sheet may be passed into an oven (6)where it is dried. The dried cast net (7) may then be separated from theembossed sheet (1) and wound up onto roller (8) where it may also beinterleaved with a release paper (9) fed from the roll (10).

In an alternative and preferred process the dried cast net is left onthe embossed sheet.

The coating head (not shown) has an adjustable doctor blade supported ona flat bed to meter the casting solution and side guides to regulate thewidth of the cast net. It is preferred that the doctor blade has a baseportion which is thick enough to span the discrete raised areas of theembossed film to prevent the doctor blade catching in the recessed areasof the film. The doctor blade and the guides can be coated or made of afluorocarbon polymer for example polytetrafluoroethylene to reducefriction against the film. Alternative coating heads using fixed orrotating rollers can also be used.

A favoured coating head comprises an adjustable doctor blade which issupported on a soft base, for example a base consisting of a movablerubber belt around two rotatable rollers, to meter the casting solution.

FIG. 2 shows discrete raised areas (11) arranged in a square pattern togive a square grid pattern of recesses (12) on the embossed castingsheet.

In FIG. 3 a section through line A--A of FIG. 2 shows the discreteraised areas (11) in the shape of truncated square pyramids and recesses(12).

The integral nets of polyurethane can be made by casting thepolyurethane in a flowable state onto a surface having a pattern ofdiscrete raised areas and interconnected recessed areas and treating thecast net to form a solid integral net. The flowable state of thepolyurethane can include solutions, dispersions, hot melts and powderswhich can be dried, coated, fused or otherwise to form a solid net. Thecasting surface may be in the form of a roller, an endless flexible beltor a length of sheet material. It is preferred that the casting surfacehas release properties to enable the formed net to be removed from thecasting surface. The pattern of the discrete raised areas andinterconnected recessed areas on the casting surface selected dictatesthe structure of the resulting net.

A preferred method of making the integral nets of polyurethane is bycasting a solution of a thermoplastic polyurethane onto a melt embossedpolyolefin sheet and drying the cast net in a hot oven.

Suitable casting solutions can contain 15% to 35% by weight ofthermoplastic polyurethane, preferably 20% to 30% by weight. Favouredcasting solutions contain 20 to 25% by weight of Estane 5702 or Estane5703 in acetone. Another favoured solution contains 25% to 30% by weightof Estane 5714F in tetrahydrofuran or mixtures of tetrahydrofuran andacetone.

Analogous procedures may be used to prepare nets from other elastomers.

The melt embossed polyolefin sheet can be made by the method given inBritish Patent Specification No. 1,055,963 A suitable embossedpolyolefin sheet has a pattern of 8 per cm raised areas in the form ofsquare truncated pyramids 1 mm. wide and 0.5 mm high with side slopingto a 60° C. conical angle and longitudinal and transverse square gridrecesses 0.25 mm wide at the base and 0.75 mm at the top.

A favoured embossed polyolefin sheet has a pattern of 6 per cm raisedareas in diagonal rows (45°) of square truncated pyramids 1.35 mm wideat their base, 0.7 mm wide at their top and 0.45 mm high with sidessloping to a 70° conical angle.

A preferred embossed polyolefin sheet has a pattern of 4 per cm raisedareas in diagonal rows (45°) of square truncated pyramids 2 mm wide attheir base, 1.425 mm wide at their top and 0.5 mm high with sidessloping to a 60° to a conical angle.

The solution of thermoplastic polyurethane can be cast onto the embossedpolyolefin surface by means of a casting head consisting of a knife overa flat bed, or knife over a roller or knife over soft bed.

Suitable conformable polyurethane films for the outer layer of thedressing can be formed by casting or spraying from solution, hot meltcoating or film extrusion in a conventional manner.

Preformed polyurethane films can be formed on a release surface forexample a silicone release coated paper.

Suitable casting solutions are described hereinbefore in relation tomaking integral polyurethane nets wound facing layers.

In an alternative process the conformable polyurethane film can beformed on the conformable hydrophilic foam layer. In an apt process thepolyurethane solution is sprayed onto the foam layer. The spraying ofthe polyurethane solution can be carried in a conventional manner forexample using an air spray gun. A suitable spray gun is model 630available from Binks Bullow Limited.

Suitable methods of making voided polyurethane-incompatible blend filmouter layers are disclosed in United Kingdom Application No. 8124250.

The adhesive layer of the outer layer can be formed by any convenientmethod including solution and emulsion coating, coating from a hot meltand by extrusion. The adhesive layer can be coated directly onto asubstrate of the dressing for example the extensible apertured non wovenfabric layer. However, it is preferred that the adhesive layer is coatedonto a release surface and the dried adhesive transferred by laminationonto the desired substrate.

The wound dressing of the invention can contain topically effectivemedicament. Most suitably the medicament is an antibacterial agent.Preferably the antibacterial agent is a broad Spectrum antibacterialagent such as a silver salt such as silver sulphadiazine, an acceptableiodine source such as povidone iodine (also called polyvinyl pyrrolidoneiodine or PVP/I), chlorhexidine salts such as the gluconate, acetate,hydrochloride or the like salts or quaternary antibacterial agents suchas benzalkonium chloride or the like.

A preferred medicament for inclusion in the dressing of this inventionis silver sulphadiazine. A further preferred medicament for inclusion inthe dressing of this invention is chlorhexidine which will normally bepresent as one of its aforementioned salts,

The medicament may be present by 0.2% to 20%, more usually from 0.3 to10% and preferably 0.5 to 5% by weight of the dressing, for example 1%,1.2% or 3% and the like. The medicament is present in the invention inthe foam layer.

It is one of the surprising features of this invention thatantibacterial agents can be incorporated into a hydrophilic polyurethanefoam and will thereafter be available to aid in maintaining the wound towhich the dressing is applied free of infection.

It is particularly surprising that medicaments such as silversulphadiazine and chlorhexidine hydrochloride and the like can beincorporated into the proto foam prior to Polymerisation since thepresence of compounds containing basic nitrogen atoms may well have beenexpected to radically change the nature of the foam which has now beenfound not to occur.

The medicament may be introduced into the foam either by incorporationprior to foaming or by incorporation into the intact foam which haspreviously been prepared.

If the medicament is to be introduced prior to foaming then themedicament must either be free of reactive moieties which would reactwith the components of the mixture to be foamed (for example it must notcontain free amino groups which could react with the isocyanatespresent) or else the medicament must be of low solubility so that itspotential reactivity is suppressed. Thus for example medicaments such assilver sulphadiazine and chlorhexidine hydrochloride are easilyincorporated into the foam by dispersing the desired amount of themeidcament into the prepolymer mixture, for example dispersing it withinthe aqueous solution of the surfactant before mixing with the isocyanatecontaining materials. Most suitably the insoluble medicaments are infinely divided form and are most preferably micronised.

It has been found that more soluble salts such as chlorhexidinegluconate cannot be incorporated in this fashion since reaction withprepolymer components can occur and a more rigid and antibacteriallyineffective foam results. Fortunately it has now been discovered thatsoluble medicaments can be included into the foam after it has beenprepared by soaking the foam in a solution of the medicament. Thus forexample a 2×2 cm dressing of this invention suspended in 50 mls of 5%w/v solution of chlorhexidine gluconate for 48 hours and dried was foundto posess antibacterial properties.

In a favoured embodiment of the invention the wound dressing may be inthe form of a compression bandage.

Most aptly the compression bandage of this invention has more than oneelastic layer. In another aspect this invention provides a low adherencycompression bandage which comprises a wound facing layer of elastomericapertured film, an intermediate layer of a conformable hydrophilic foamand an outer layer of an elastomeric continuous moisture vapourpermeable film.

Most aptly the elastomeric apertured film is a net as hereinbeforedescribed.

In preferred compression bandages of this invention the integral netwound facing layer, the intermediate foam layer and the outer film layerare made of polyurethane as hereinbefore described.

Low adherency compression bandages of the invention can be used to coverskin grafts. In this application the bandage may absorb exudate from thegraft surrounds and at the same time exert an even pressure over thegraft site which can protect against hypertrophic scarring.

The low adherency compression bandages of this invention may also beused in the treatment of ulcers, for example leg ulcers.

The bandages of this invention will normally be presented as rolls offrom 1 to 4 meters length and 5 to 20 cms width.

After fabrication the wound dressing of this invention can be washedwith water to remove excess surfactant and then dried.

Surprisingly the method of drying has been found to greatly affect theappearance of the dressing.

Some air dried dressings have been found to curl. In some cases theouter film has puckered thereby giving the film a grained appearance andtexture of skin.

Dressings autoclaved using vacuum drying cycle have been found to tendto remain flat. The outer film layer has a puckered surface. Partiallydried dressings, that is dressings containing low levels of residualwater absorbed into the foam polymer but not into foam air spaced havebeen discovered to be flat. This unanticipated effect is rendered evenmore useful since the dressing will remain flat if protected againstloss of water, for example if packaged in a water proof pouch such as analuminium foil pouch. In such partially dried dressings the film and netare not puckered.

Dressings containing residual amounts of water in this way have beenfound to have a pleasant cooling feel.

As previously indicated hereinbefore the dressings of this invention maybe adapted to release an antibacterially effective amount of ananti-bacterial agent into the wound covered by the dressing. Thus in analternative aspect this invention provides a method of treating a woundso as to aid in rendering or maintaining it free of infection whichcomprises contacting the wound with a dressing of this invention adaptedto release an antibacterial agent. Most aptly this aspect of theinvention is employed in rendering or maintaining burns free ofinfection. The antibacterial agent present is favourably a silver saltsuch as silver sulphadiazine or a chlorhexidine salt such aschlorhexidine hydrochloride or a mixture thereof. Preferably theantibacterial agent present is silver sulphadiazine.

The absence of fibres in the dressing enhances the non adherentproperties of the wound dressings of the invention.

Description Preparation of Integral Polyurethane Net

A solution containing 30% by weight of Estane 5714F1 in tetrahydrofuranwas cast into the recesses of a 15 cm wide melt embossed high densitypolyethylene sheet by means of the blade over flat bed spreadingtechnique. The sheet had a melt embossed pattern of 8 per cm raisedareas in the form of square truncated pyramids 1mm wide at their baseand 0.5 mm high with sides sloping to a solid conical angle of 60° C.The wet cast net on the embossed film was dried by passage through a hotair circulating oven at a temperature of 90° C. to 100° C. for twominutes. The dried cast net was separated from the embossed film andwound onto a roller interleaved with a double sided silicone releasepaper.

The resultant cast integral of elastomeric polyurethane net had thefollowing properties: Weight 40 gsm; thickness 100-125 microns; aperturesize 0.3 to 0.4 mm; tensile strength (g/2.5 cm wide), machine direction800±51, transverse direction 664±57, elongation at break %, machinedirection 389±24, transverse direction 374±24.

Preparation of Integral Polyurethane Diamond Pattern Net (6apertures/cm).

The net was cast in the same manner as the preceeding square net exceptthat the casting sheet had a melt embossed pattern of 6 per cm raisedareas in diagonal rows (45°) of square truncated pyramids 1.35 mm wideat their base and 0.45 mm high with sides sloping to a conical angle of70°

Example 1 Preparation of a low adherency wound dressing

A mixture of Brij 72 (22.5 g of 2% aqueous emulsion) and Pluronic F87(0.5 ml of a 10% aqueous solution) was added to Hypol FHP3001 (15 g) ina beaker and thoroughly mixed by stirring with a metal spatula until theHypol was uniformly dispersed (20 seconds). The foaming mixture waspoured into a 15 cm wide brass hand spreader box set at a gap of 1.8 mmabove a 25 micron thick cast Estane 5714F1 film. The spreader box wasthen drawn by hand along the film surface to leave a foam layer on thefilm The cast integral polyurethnae net of the description was thenlaminated to the foam by placing the net (smooth surface uppermost) ontothe setting foam 3 minutes to 3.25 minutes after the spreading hadcommenced. The foam layer was free of large craters and was well bondedto the net.

Alternatively the wound dressing can be made in a similar manner bycoating the loam onto the integral polyurethane net (preferably on theembossed film carrier) and laminating the film to the setting foam.

Sample wound dressings of Example 1 and the comparison hydrophilicpolyurethane foam were washed with distilled water and dried at 40° C.for 12 hours before being tested for wound adherency.

Example 2

Brij 72 (30 g as a 2.5% aqueous solution) was added to Hypol FHP 3001(20 g) in a beaker and mixed by stirring with a metal spatula and thenwith a mechanical stirrer until the Hypol was uniformly dispersed(approximately 20 seconds). The foaming mixture was cast onto a 6aperture/era diamond pattern net as described above on its embossed filmcarrier by means of a blade over flat bed coating head set at a gap of0.1 mm. A 25 micron thick film (Estane 5714F1) on a silicone releasepaper was laminated to the foam surface with light manual pressure tothe foam surface while it was still tacky (approximately 3 minutes afterspreading). After 15 minutes the embossed film and silicone coatedrelease papers were removed from the net and film surfaces of the stripand the strip cut into 30 cm×15 cm wound dressings. The dressings werewashed in two changes of 1 liter of distilled water and dried in air.

Example 3

Wound dressings were prepared in the same manner as Example 2 using agap setting of 0.5 mm instead of 0.l mm.

Example 4

Wound dressings were prepared in the same manner as Example 2 using agap setting of 1.0 mm instead of 0.l mm.

Example 5

The wound dressings of Example 3 were autoclaved at 116° C. for 30minutes followed by a vacuum drying cycle.

Example 6

The wound dressings of Example 4 were autoclaved in the same manner asExample 5.

Example 7

The dressings of Example 4 were washed but only partially dried bypadding with an absorbent towel.

The following Examples 8 to 12 the preparation of of medicated wounddressings of the invention.

Example 8

Wound dressings were prepared in the same manner as Example 3 exceptthat silver sulphadiazine powder (0.2 g) was blended into the Brij 72emulsion with a high speed shear mixer prior to the addition of HypolFHP 3001.

Example 9

Wound dressings were prepared in the same manner as Example 8 using 1 gof chlorhexidine hydrochloride powder instead of silver sulphadiazinepowder (0.2 g).

Example 10

Wound dressings prepared as in Example 3 were soaked for 10 minutes in atray containing an aqueous solution of chlorhexidine gluconate (5%weight/volume) and air dried.

Example 11

Wound dressings were prepared in the same manner as Example 10 using anaqueous solution of chlorhexidine acetate (5% weight/volume) instead ofan aqueous solution of chlorhexidine gluconate.

Example 12

Wound dressings were prepared in the same manner as Example 10 using anaqueous solution of povidone iodine (10% weight/volume) instead of anaqueous solution of chlorhexidine gluconate.

Example 13

Wound dressings were prepared in the same manner as Example 7 using 20 ginstead of 30 g of Brij 72.

Example 14

Wound dressings were prepared in the same manner as Example 7 using 40 ginstead of 30 g of Brij 72.

Example 15

Wound dressings were prepared in the same manner as Example 7 with 0.5ml of Pluronic L64 (10% aqueous solution) added to the surfactantemulsion.

Example 16

Wound dressings were prepared in the same manner as Example 15 usingPluronic F68 in place of Pluronic L64.

Example 17

Wound dressings were prepared in the same manner as Example 15 usingPluronic F108 instead of Pluronic L64.

Example 18

Wound dressings were prepared in the same manner as

Example 1 using Brij 72 (30 g as a 2.5% aqueous emulsion) instead of amixture of Brij 72 (22.5 g as a 2% aqueous emulsion) and Pluronic F87 asa 10% aqueous solution).

Example 19

Wound dressings were prepared as Example 18 using a mixture of Brij 72and a Pluronic P75 (0.5 ml as a 10% aqueous solution).

Example 20

Wound dressings were prepared in the same manner as Example 1 using acast polybutadiene (ref RB830) net (8 apertures per cm) instead of apolyurethane net.

Example 21

Preparation of a Compression Bandage

A bandage strip was made in the same manner as the wound dressing stripof Example 2 using a blade gap setting of 0.5 mm. The strip was washedin distilled water and dried in air. A 1 cm wide bandage had thefollowing stress-strain properties:

    ______________________________________                                               Strain %                                                                             Stress g/cm                                                     ______________________________________                                               25     360                                                                    50     610                                                                    100    800                                                             ______________________________________                                    

To illustrate the compression properties of the bandage the pressure (P)exerted on a-limb of radius (R) by a single layer of the bandagestretched by different amounts was calculated from the stress-strainproperties above using the equation p=F/R where F is the force (g/cm) tostretch the bandage by a given amount.

    ______________________________________                                                 stretched length                                                              (% of original                                                                length)     g/cm.sup.-2                                                                            .sup.P mmHg                                     ______________________________________                                        Arm 10 cm in                                                                             125           72       54                                          diameter   150           122      92                                                     200           160      120                                         Leg 20 cm in                                                                             125           36       27                                          diameter   150           61       46                                                     200           80       60                                          ______________________________________                                    

Antibacterial properties of the Medicated Dressings

15 mm diameter samples of medicated wound dressings of Examples 9 to 14were moistened and placed on agar plates seeded with staphyloccocusaureus or bacillus subtilis and incubated for 24 hours at optimumtemperature for growth. The zones of inhibition produced by diffusion ofthe medicament from the dressings were measured.

    ______________________________________                                                             Zone diameter                                                                       Staph                                              Example Medicament         aureus   B. subtilis                               ______________________________________                                         8      silver sulphadiazine   30     29.5                                     9      chlorhexidine Hcl      23.8   21.6                                    12      povidone iodine        29.6   19.4                                    11      chlorhexidine acetate  29.0   32.0                                     10*    chlorhexidine gluconate                                                                        (1)   22.6   23.5                                                             (2)   19.8   19.1                                                             (3)   16.8   16.0                                    control (no medicament)        0      0                                       ______________________________________                                         *consecutive tests on the same sample to measure the effect of the            sustained release of the medicament from the dressing.                   

Example 22 Preparation of Integral Diamond Pattern Net (4 apertures/cm)

A solution containing 20% by weight of Estane 5714F in 60/40 (weight byweight) mixture of Tetrahydrofuran acetone was cast into the recesses ofa 15 cm wide melt embossed high density polyethylene sheet by means of ablade over soft bed coating technique. The sheet had a melt embossedpattern of 4 per cm raised areas in diagonal rows (45° ) of squaretruncated pyramids 2 mm wide at their base, 1.42 mm wide at top and 0.5mm high with sides sloping to a conical angle of 60° The wet cast net inthe embossed film was dried by passage through a hot air oven attemperature of 80° C. for two minutes.

The net had a weight per square meter of 33 g and had 4 per cm aperturesof approximately 1.4 mm in size.

Preparation of the absorbent layer

Using a two component dispensing unit (Vario-mix supplied by ProdefEngineering Limited), a foaming mixture was formed by mixing Hypol FHP2002 and Brij 72 (2% aqueous solution) in the ratio of 1:2.25. Thefoaming mixture was fed into the coating head by means of an outputnozzle in the form of a 15 cm `fish tail die` and coated onto the castpolyurethane net (on embossed film) by means of a knife over rollercoating head set at a gap of 1 min. The cast foam was dried by passagethrough an air circulating oven at a temperature of 50° C. for 5minutes.

Preparation of the Conformable Film Outer Layer

A solution containing 2% by weight a polyurethane of (Estane 5714F) in a60/40 (weight by weight) mixture of tetrahydrofuran/acetone was handsprayed onto the foam surface of the composite foam/net strip using anair spray unit (model 630 supplied by Binks Bullow Limited) and dried bypassage through an air circulating oven heated to a temperature of 70°C.

The polyurethane coating was found to be continuous and had a weight persquare meter of approximately 30 grams.

The embossed film was then removed from the three layer composite stripand the strip cut into dressings of suitable size for adherency testing.

Example 23

A foam-net composite strip on embossed film was made in the same manneras example 22.

An extensible apertured non woven fabric (Bemliese GS 204) coated with acontinuous layer of a moisture vapour transmitting acrylate copolymeradhesive (30 g/m²) was laminated to the foam side of the composite stripto form a conformable outer layer.

The adhesive was a copolymer of 47 parts by weight n-butyl acrylate, 47parts by weight of 2-ethyl-hexyl acrylate and 6 parts by weight ofacrylic acid having an intrinsic viscosity of 1.9 dl/g polymerised inacetone according to the method given in United Kingdom Application No.8106707.

The embossed film was then removed from the composite strip and thestrip cut into suitable sized dressings of the invention.

Example 24

Wound dressings were prepared in the same manner as Example 22 using acast polybutadiene (Ref RB 830) net (6 apertures/cm) instead of apolyurethane net.

Adherency Testing

A group of 9 guinea pigs were clipped and depilated over thethorax/abdominal region. Two partial thickness burns, 2.5 cm in diameterwere created, one on each flank, by contact with hot water at 65° C. for15 seconds. The skin was dried and the burnt area scraped with a scalpelblade to remove the epidermis. Histological sections have previouslyshown that this technique separates the skin between the epidermis anddermis. Dressings were applied to the wound with a margin ofapproximately 1 cm all round. Crepe bandage was wrapped round the animaland secured with an elastic adhesive bandage. The dressings were left inplace for 24 hours. The animals were then sacrificed with anintracardiac injection of pentobarbitone and full thickness sections ofskin with a generous margin around the wound was taken from the animal.The dressings were then peeled from the skin biopsy using an Instrontensile testing machine at a peel speed of 5 inches per minute. Themean, maximum and integrated total force of removal (g) were recorded.The product of Example 1 required 210 g for removal.

The energy for removal (milli-joules) of the dressing from the skinbiopsy can be calculated from the peel adhesion results. The dressingsof Examples 3 and 22 had an energy removal which was approximately halfthat of a Melolin* dressing (energy of removal approximately 6.5mJ/cm²).

What is claimed is:
 1. A conformable low adherency wound dressing whichcomprises(a) a low adherency wound facing layer 0.05 to 2.5 mm thickwhich is a conformable elastomeric apertured net; (b) an absorbent layercomprising a conformable hydrophilic foam 0.5 to 20 mm thick; and (c) acontinuous conformable moisture vapour permeable outer film layer whichacts as a bacterial barrier and has a moisture vapour transmission rateof from 300 to 5000 grams/square meter/24 hours at 37° C. at 100% to 10%relative humidity difference, said wound facing layer and said outerlayer being laminated to opposing faces of said absorbent layer over theoperative area of the dressing.
 2. A dressing according to claim 1, inwhich the conformable elastomeric net comprises an integral net.
 3. Adressing according to claim 1, in which the conformable elastomeric nethas 4 to 40 apertures per cm.
 4. A dressing according to claim 1, inwhich the conformable elastomeric net comprises a polyurethane.
 5. Adressing according to claim 1, in which the continuous conformable layerhas a moisture vapour transmission rate of 500 to 2000 grams/squaremeter/24 hours at 37° C. at 100% to 10% relative humidity difference. 6.A dressing according to claim 1, in which the continuous moisture vapourtransmitting conformable film layer comprises a polyurethane.
 7. Adressing according to claim 1, in which the conformable outer film layercomprises a moisture vapour transmitting adhesive layer.
 8. A dressingaccording to claim 1, in which the conformable hydrophilic foam is afoam of hydrophilic polymer.
 9. A dressing according to claim 8, inwhich the hydrophilic polymer is hydrophilic polyurethane.
 10. Adressing according to claim 1, in which the hydrophilic foam comprisesan open cell foam with a cell size of 50 to 500 microns.
 11. A dressingaccording to claim 1, in which the hydrophilic foam comprises an opencell foam in which 30% to 60% of the total area of the cells aremembrane openings.
 12. A sterile dressing according to claim 1, within abacteria impervious package.